A series of high affinity cerebral binding systems are being developed with the purpose of utilizing this assay as a routine neurotoxicological test. Ligand-receptor interactions that have been characterized include those for serotonin, GABA, diazepam, glycine, muscarinic acetylcholine, dopamine, and the alpha and beta adrenergic sites. Enkephalin receptors will also be studied. Regional distribution and specificity determinations have been made for these receptors. The developmental profile of receptors is being assayed in rats and chicks. The extent to which circadian and other environmental factors influence receptor formation and maintenance is under study. Specific increases in the striatal dopamine receptor have been found in acrylamide-treated rats after single or repeated dosing. However, prenatal and neonatal exposure to acrylamide results in a reduction of striatal dopamine receptors. This receptor is depressed in adult mice prenatally exposed to polychlorinated biphenyls. Kepone-treated rats exhibit more general receptor changes. Manganese and tin exposed rats are also being tested. Receptors have been shown to be inhibited in vitro by low concentrations of tri-n-butyl lead acetate but not by lead acetate. Methyl mercuric chloride is generally less inhibitory than inorganic mercuric chloride.